Vibration isolation systems are used in a wide variety of applications to reduce transmission of mechanical vibrations generated by noisy components or carried by the environment to a sensitive device. Examples include isolation of optical benches from floor-borne seismic disturbances, isolation of a car or airplane body from engine vibrations, or suspension systems of vehicles.
Isolation is achieved by inserting soft mechanical links ("isolators") between the subsystem containing the source of the disturbances and the subsystem to be isolated. Based on the relative sizes of these subsystems, two classes of isolation systems can be distinguished.
In the first situation, the environment is isolated from vibrations created by a piece of machinery. A typical example is the isolation of a car body from vibrations caused by the engine, or isolation of a facility floor from vibrations induced by rotating machinery (compressors, presses, etc.). In the other, a sensitive device is protected from disturbances carried by its supporting structure. Isolation of an optical bench from floor borne vibrations is a common example.
In both cases, the effectiveness of the isolation system can be examined in terms of transmissibility functions, T, in the frequency domain. In the first class of systems, these transmissibilities can be expressed as ratios of excitation forces to forces transmitted to the floor. In the second class, they are usually expressed as ratios of component of floor motion to component of device motion. Mixed formulations are also possible. Whatever exact definition is used for T, its magnitude typically can be represented by a graph with three regions. At "low" frequencies, resonant peaks corresponding to the suspension modes are observed. There is no isolation in this band (in fact there is amplification). At the "high" end of the spectrum, flexible modes of the device and/or supporting structure themselves produce other resonant peaks. In a properly designed isolation system, those two frequency ranges are separated by a wide "isolation band" where transmission decays rapidly with frequency f (about -40 dB/decade for a lightly damped, single stage system). A properly designed system will "see" most of the disturbance energy occur in this band.
In the isolation band, the transmissibility is a direct function of the "softness" of the isolator mounts. This "softness" can be expressed by a characteristic deflection parameter .delta..sub.max. For usual applications, where the isolators also support the weight of the device, .delta..sub.max is defined as ##EQU1## where P.sub.max is the maximum static load that can be supported by the isolator and k is the stiffness of the isolator. Note that k may be a frequency-dependent quantity (as is the case with rubber isolators for example).
At lower frequencies however, resonances of the suspension dominate the transmissibility. To properly dampen the response to transients and to limit the amplitude of the response to disturbances at the resonance frequencies, isolation systems must also provide some damping. That damping is usually expressed in terms of the quality factor (Q) of a resonant mode. The Q defines the dynamic amplification at resonance (ratio of dynamic to static response) and is equal to ##EQU2## where .eta. is the loss factor and .zeta. is the critical damping ratio of the mode considered. The Q's of the suspension are directly controlled by the loss factors of the isolators. Examples of high loss isolators include rubber supports or spring/damper combinations. Note also that viscous damping (dashpots in vehicle suspensions for example) is less than ideal in isolation applications because of the stiffening effect at high frequencies, which leads to loss of isolation performance. Viscoelastic damping is in general preferable because of lesser stiffening.
In short, soft mounts for high performance isolation systems must have the following characteristics: large characteristic deflections in the isolation range (typically above 1 to 15 Hz), and high loss in the resonance region (typically below 1 to 15 Hz). Other desirable characteristics are compactness and simplicity, low drift, good aging characteristics in a variety of environments, and low outgassing potential for high vacuum applications.
The present invention consists of a novel way of combining the desired spring and damping effects into a single, self-contained isolator. A metal spring (coil or torsion rod), has embedded viscoelastic damping treatments. The load carrying metal element offers advantages of potentially very large characteristic deflection .delta..sub.max and negligible drift or aging. This contrasts with rubber isolators which have relatively poor characteristic deflections (i.e. they are relatively stiff for a given load capacity), considerable long term drift, and aging problems. The inclusion of self-contained damping treatments provides the required damping at low frequency with a much simpler system than combinations of separate spring and damper units. The concept also allows for a continuous, sealed metal envelope that traps outgassing materials for use in high vacuum systems.
Damping is obtained through a novel application of the well known constrained layer damping (CLD) technique where a thin layer of viscoelastic material is sandwiched between two layers of structural material in such a way that deformations in the structure induced large shear strains in the damping layer. The novelty consists of the application of CLD to the damping of torsional deformations of a wire-like structure. The torsional deformations result from direct torsional loading (as in the case of a torsion rod) or occur in response to traction or compression loading of coil springs.
Accordingly, it is an object of the present invention to provide metal springs and torsion rods with damping characteristics from embedded constrained layer viscoelastic damping treatments that are suitable for transient and low frequency responses in isolation systems, while providing the large characteristic deflections required for isolation effectiveness.
Another object of the present invention is to provide springs with entirely sealed damping treatments as required for use in high vacuum systems.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.